We investigated coordination between rhythmic flexion-extension (FE) and supination-pronation (SP) movements at the elbow joint-complex using a 2-degrees of freedom motorized robot arm. Most participants spontaneously adopt an inphase coordination pattern (i.e. forearm supination is synchronized with elbow flexion) in this task. The goal of the present study was to assess whether this behaviour is due to patterns of efference that are optimized to exploit the biomechanical advantage bestowed by the bifunctional nature of the Biceps Brachii (BB) muscle, which acts to both flex the elbow and supinate the forearm. Electromyograms of BB and two other arm muscles were recorded during an experiment in which participants had to produce oscillatory FE and SP movements either in isolation or in combination (at a freely chosen phase relationship). In a normal (neutral) dynamic context, BB was more active during combined (in phase) FE and SP than when these movements were performed in isolation. The motorized robot arm was used to generate an alternative dynamic context which promoted the opposite (i.e., anti-phase) pattern of coordination. In these circumstances, however, the in-phase mode of coordination remained predominant. It is proposed that the synergistic relations between the muscles engaged in this task are not sufficiently flexible to permit exploitation of the bifunctional characteristic of BB in altered dynamical contexts
